In optical data transmission, data values may be transmitted by means of a phase modulated optical transmission signal. The optical transmission signal is generated, by modulating the phase of an optical carrier signal, which possesses a carrier frequency, in dependence on the transmitted data values and in accordance with a constellation diagram of a respective phase-shift keying (PSK) modulation method. Each point of the constellation diagram represents a finite set of data values that are to be transmitted, wherein the set of data values is called a data symbol. A data symbol is represented by a corresponding constellation point of a constellation diagram, wherein the constellation point has a corresponding symbol phase value. Depending on the data symbols that are to be transmitted, respective constellation points and symbol phase values are derived. A signal that corresponds to a symbol phase value can be interpreted as a superposition of an inphase signal component and a quadrature signal component, wherein                both signal components have a same frequency,        the inphase signal component has a phase that is equal to zero, while the quadrature signal component has a phase that differs from the phase of the inphase signal component by π/2, and        the respective amplitudes of the two signal components determine the corresponding symbol phase value.        
The phase of the optical carrier signal is modulated, such that it corresponds to the derived symbol phase values representing the respective data symbols. The generated optical signal is then transmitted over an optical channel.
At a receiving side, the transmitted optical signal is received from the optical channel and demodulated, using a coherent reception scheme: the received optical signal is mixed with a local optical signal that possesses the carrier frequency and a phase, which is ideally equal to the phase of the optical carrier signal used at the transmitting side. The local optical signal is provided by a local oscillator present at the receiving side. The mixing of the optical signals with the local optical signal yields an inphase signal component and a quadrature signal component in the electrical domain, wherein these signal components can be superposed to form a received signal in the electrical domain.
Using a hard decision detection scheme, it may be decided for that point of the constellation diagram, whose symbol phase value is most similar to the phase value represented by the received electrical signal. Thus, received data can be derived from the derived signal components.
For transmitting data values at a higher data rate, the concept of phase-modulation used in PSK may be extended, by not only modulating the phase of the optical signal but also modulating the amplitude of the optical signal. Such a combined phase and signal modulation is also known as Quadrature Amplitude Modulation (QAM).